Hydrangea kawagoeana Koidz. — Tokara hydrangea

Introduction

Tokara hydrangea, Hydrangea kawagoeana, is a small woody shrub of the northern and central Ryukyu island chain and one of the insular Japanese members of Section Kawagoeanae. The section was established by Yang et al. (2024) after expanded molecular sampling showed that several plants formerly grouped loosely within Section Chinenses instead form a distinct lineage containing H. kawagoeana, H. luteovenosa, H. grosseserrata, and several Taiwanese and more southerly relatives. Morphologically, the two sections remain difficult to separate consistently, making Kawagoeanae a particularly clear example of molecular evidence exposing evolutionary structure that had been obscured by broadly similar shrub morphology.

Hydrangea kawagoeana is typically a shrub about 1–2 m tall with opposite, rather thick leaves and terminal inflorescences bearing white enlarged marginal flowers around much smaller fertile flowers. Japanese botanical sources describe it particularly from forest and streamside habitats in the Tokara and Amami island systems, with records extending southward within the Ryukyus. The precise published range varies somewhat among databases, in part because older treatments frequently included the Yakushima plant now accepted by Tree TSAR as the distinct species H. grosseserrata.

The relationship with H. grosseserrata is therefore central to understanding this species. Major international backbones still treat the Yakushima plant as H. kawagoeana var. grosseserrata, but nuclear MIG-seq data recovered substantial differentiation between sampled kawagoeana and grosseserrata, with an FST of 0.395. That value falls within the range observed among other recognized species in the same molecular study. The ITS analysis also recovered the two together as a strongly supported pair, showing that they are close relatives while remaining genetically differentiated. Tree TSAR consequently accepts H. kawagoeana and H. grosseserrata as separate species rather than varieties of one species. (Hirota et al. 2022)

The distinction is not purely molecular. Japanese botanical accounts characterize Tokara hydrangea as having somewhat shorter petioles and thicker leaf blades than Yakushima hydrangea. The differences are modest, which explains the long history of treating the two as varieties, but in combination with island geography and genome-wide differentiation they support recognition of two evolutionary units.

Horticultural and Agricultural Uses

Hydrangea kawagoeana has little documented presence in Western horticulture and appears to remain primarily a plant of specialist collections and Japanese native-plant cultivation. Trees and Shrubs Online notes that it is probably not represented in Western gardens to any meaningful degree, in contrast with the better-known H. luteovenosa and H. scandens.

Its horticultural interest lies in its compact stature, thick foliage, relatively small lacecap inflorescences, and early-summer flowering. More importantly, wild-source material would provide living representation of a geographically restricted Ryukyu lineage that has rarely entered horticultural circulation. Provenance should be retained carefully, particularly because plants historically circulated under broad kawagoeana or related names may not correspond to the narrowly circumscribed species accepted here.

Conservation Concerns

Tokara hydrangea is geographically restricted even though it may be locally frequent. Japanese sources document it from the Tokara and Amami island systems and report it as common in parts of Tokunoshima, while Okinawa has treated relevant populations as Endangered. Kagoshima conservation listings emphasize its distributional significance rather than placing the species as a whole in a national threatened category.

Kew's automated extinction-risk model predicts H. kawagoeana to be not threatened, but that prediction is not a formal Red List assessment and is complicated by Kew's broader taxonomic treatment, which includes H. grosseserrata as an infraspecific taxon. Conservation assessments conducted under different circumscriptions should therefore not be transferred directly to the narrower Tree TSAR species concept.

Maintaining populations across multiple islands is particularly important because geographic fragmentation is part of the evolutionary structure of Section Kawagoeanae. Future population-genomic sampling across the Tokara and Amami chains could determine whether H. kawagoeana itself contains additional island-level structure or gene flow with neighboring sectional relatives.

Additional Information

Plants of the World Online — Hydrangea kawagoeana (opens in a new tab): Current international backbone treatment, distribution, and nomenclature; note that POWO retains H. grosseserrata at varietal rank.

National Museum of Nature and Science — Ryukyu plant database (opens in a new tab): Japanese distribution and regional conservation information for Tokara hydrangea.

iNaturalist — search for Hydrangea kawagoeana (opens in a new tab): Photographic occurrence records; cultivated observations and taxonomic identifications should be interpreted cautiously.

References and Further Reading

Hirota SK, Yahara T, Fuse K, Sato H, Tagane S, Fujii S, Minamitani T, Suyama Y (2022) Molecular phylogeny and taxonomy of the Hydrangea serrata complex (Hydrangeaceae) in western Japan, including a new subspecies of H. acuminata from Yakushima. PhytoKeys 188: 49–71. https://doi.org/10.3897/phytokeys.188.64259 (opens in a new tab)

Iwatsuki K, Boufford DE, Ohba H (eds.) (2001) Flora of Japan, vol. IIb. Kodansha, Tokyo. https://www.kspub.co.jp/book/detail/1546059.html (opens in a new tab)

Yang X, Xue T, Gao T, Liang Y, Smets EF, Gadagkar SR, Yu S (2024) An updated infrageneric classification based on phylogenomics and character evolution in Hydrangea (Hydrangeaceae). Taxon 73(2): 503–518. https://doi.org/10.1002/tax.13158 (opens in a new tab)